Update CHANGELOG

Author: carmocca

CHANGELOG.md

@@ -185,6 +185,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Validation is now always run inside the training epoch scope ([#7357](https://github.com/PyTorchLightning/pytorch-lightning/pull/7357))
 
 
+- `ModelCheckpoint` now runs at the end of the training epoch by default ([#8389](https://github.com/PyTorchLightning/pytorch-lightning/pull/8389))
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 - `EarlyStopping` now runs at the end of the training epoch by default ([#8286](https://github.com/PyTorchLightning/pytorch-lightning/pull/8286))